Contents

Origin of name

The name of the Pleiades comes from Ancient Greek. It probably derives from plein ('to sail') because of the cluster's importance in delimiting the sailing season in the Mediterranean Sea: 'the season of navigation began with their heliacal rising'.[9] However, the name was later mythologised as the name of seven divine sisters, whose name was imagined to derive from that of their mother Pleione,
effectively meaning 'daughters of Pleione'. However, in reality the
name of the star-cluster almost certainly came first, and Pleione was
invented to explain it.[10]

The Nebra sky disk, dated circa 1600 BC. The cluster of dots in the upper right portion of the disk is believed to be the Pleiades.

The Babylonian star catalogues
name the Pleiades MUL.MUL or "star of stars", and they head the list of
stars along the ecliptic, reflecting the fact that they were close to
the point of vernal equinox around the 23rd century BC. The earliest known depiction of the Pleiades is likely a bronze age artifact known as the Nebra sky disk, dated to approximately 1600 BC. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey. They are also mentioned three times in the Bible (Job 9:9 and 38:31, as well as Amos 5:8). Some scholars of Islam suggested that the Pleiades (ath-thurayya) are the star mentioned in the sura (chapter) Najm of the Quran.
In Japan, the constellation is mentioned under the name Mutsuraboshi ("six stars") in the 8th century Kojiki and Manyosyu documents. The constellation is also known in Japan as Subaru (“unite”) and is depicted in the logo and name of the Subaru automobile company. The Persian equivalent is Nahid (pronounced "Naheed").
The rising of the Pleiades is mentioned in the Ancient Greek text Geoponica.[11] The Greeks oriented the Hecatompedon temple of 550 BC and the Parthenon of 438 BC to their rising.[12]
The rising of the Pleiades before dawn (usually at the beginning of
June) has long been regarded as the start of the new year in Māori culture, with the star group being known as Matariki. The rising of Matariki is celebrated as a midwinter festival in New Zealand.[13] In Hawaiian culture the cluster is known as the Makali'i and their rising shortly after sunset marks the beginning of Makahiki, a 4-month time of peace in honor of the god Lono.

Animation of proper motion in 400,000 years (cross-eyed viewing ). Due to technical limitations on thumbnails, you must click through to the actual image to see the animation.

Galileo Galilei was the first astronomer to view the Pleiades through a telescope.
He thereby discovered that the cluster contains many stars too dim to
be seen with the naked eye. He published his observations, including a
sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610.
The Pleiades have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell
calculated in 1767 that the probability of a chance alignment of so
many bright stars was only 1 in 500,000, and so correctly surmised that
the Pleiades and many other clusters of stars must be physically
related.[14] When studies were first made of the stars' proper motions,
it was found that they are all moving in the same direction across the
sky, at the same rate, further demonstrating that they were related.Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe
cluster, Messier's inclusion of the Pleiades has been noted as curious,
as most of Messier's objects were much fainter and more easily confused
with comets—something that seems scarcely possible for the Pleiades.
One possibility is that Messier simply wanted to have a larger catalogue
than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.[15]Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.[16][17][18]

Distance

The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder.
As the cluster is so close to the Earth, its distance is relatively
easy to measure and has been estimated by many methods. Accurate
knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram
for the cluster, which, when compared to those plotted for clusters
whose distance is not known, allows their distances to be estimated.
Other methods can then extend the distance scale from open clusters to
galaxies and clusters of galaxies, and a cosmic distance ladder can be
constructed. Ultimately astronomers' understanding of the age and future
evolution of the universe is influenced by their knowledge of the
distance to the Pleiades. Yet some authors argue that the controversy
over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder
can (presently) rely on a suite of other nearby clusters where
consensus exists regarding the distances as established by Hipparcos and
independent means (e.g., the Hyades, Coma Berenices cluster, etc.).[3]
Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 parsecs by measuring the parallax
of stars in the cluster—a technique that should yield the most direct
and accurate results. Later work consistently argued that the Hipparcos
distance measurement for the Pleiades was erroneous.[3][4][5][19][20][21] In particular, distances derived to the cluster via the Hubble Space Telescope and infrared color-magnitude diagram fitting favor a distance between 135–140 pc;[3][19] a dynamical distance from optical interferometric observations of the Pleiad double Atlas favors a distance of 133-137 pc.[21] However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc, and challenged the dissenting evidence.[2] Recently, Francis and Anderson[22] proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean, and gave a Hipparcos parallax distance of 126 pc, and photometric distance 132 pc based on stars in the AB Doradus, Tucana-Horologium moving group and Beta Pictoris
moving groups, which are similar in age and composition to the
Pleiades. Those authors note that the difference between these results
can be attributed to random error.
The latest result (August, 2014)[23] used very long baseline radio interferometry (VLBI) to determine a distance of 136.2 ± 1.2 pc, conclusively showing "that the Hipparcos measured distance to the Pleiades cluster is in error."

Composition

X-ray
images of the Pleiades reveal the stars with the hottest atmospheres.
Green squares indicate the seven optically brightest stars.

The cluster core radius is about 8 light years and tidal radius
is about 43 light years. The cluster contains over 1,000 statistically
confirmed members, although this figure excludes unresolved binary stars.[24] It is dominated by young, hot blue stars,
up to 14 of which can be seen with the naked eye depending on local
observing conditions. The arrangement of the brightest stars is somewhat
similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses.[24]
The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun's mass, not heavy enough for nuclear fusion
reactions to start in their cores and become proper stars. They may
constitute up to 25% of the total population of the cluster, although
they contribute less than 2% of the total mass.[25]
Astronomers have made great efforts to find and analyse brown dwarfs in
the Pleiades and other young clusters, because they are still
relatively bright and observable, while brown dwarfs in older clusters
have faded and are much more difficult to study.

Age and future evolution

Ages for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolution.
Using this technique, ages for the Pleiades of between 75 and 150
million years have been estimated. The wide spread in estimated ages is a
result of uncertainties in stellar evolution models, which include
factors such as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, resulting in higher apparent ages.
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions. Brown dwarfs can retain their lithium, however. Due to lithium's very low ignition temperature of 2.5 million kelvin,
the highest-mass brown dwarfs will burn it eventually, and so
determining the highest mass of brown dwarfs still containing lithium in
the cluster can give an idea of its age. Applying this technique to the
Pleiades gives an age of about 115 million years.[26][27]
The cluster is slowly moving in the direction of the feet of what is currently the constellation of Orion.
Like most open clusters, the Pleiades will not stay gravitationally
bound forever. Some component stars will be ejected after close
encounters with other stars; others will be stripped by tidal
gravitational fields. Calculations suggest that the cluster will take
about 250 million years to disperse, with gravitational interactions
with giant molecular clouds and the spiral arms of our galaxy also hastening its demise. [28]

Reflection nebulosity

Under ideal observing conditions, some hint of nebulosity may be seen
around the cluster, and this shows up in long-exposure photographs. It
is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.
It was formerly thought that the dust was left over from the formation
of the cluster, but at the age of about 100 million years generally
accepted for the cluster, almost all the dust originally present would
have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.
Studies show that the dust responsible for the nebulosity is not
uniformly distributed, but is concentrated mainly in two layers along
the line of sight to the cluster. These layers may have been formed by
deceleration due to radiation pressure as the dust has moved towards the stars.[29]

Brightest stars

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin (Πλειάδες), though of uncertain etymology. Suggested derivations include: from πλεῖν plein, "to sail," making the Pleiades the "sailing ones"; from πλέος pleos, "full, many"; or from πελειάδες peleiades, "flock of doves." The following table gives details of the brightest stars in the cluster:

Possible planets

Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of the cluster's stars – HD 23514,
which has a mass and luminosity a bit greater than that of the Sun, is
surrounded by an extraordinary number of hot dust particles. This could
be evidence for planet formation around HD 23514.[30]